The present invention relates to a reflective liquid crystal display using thin film transistors and a method of forming the same.
A conventional reflective liquid crystal display using thin film transistors disclosed in the Japanese laid-open patent publication No. 5-224186 will be described with reference to FIG. 1. The reflective liquid crystal display is formed on an insulation substrate 1 such as a glass substrate. A gate electrode 2 is selectively formed on the insulation substrate 1. A gate insulation film is provided which extends over the gate electrode 2 and the top surface of the insulation substrate 1. A semiconductor layer 4 is selectively provided over a part of the gate insulation film 3 extending over the gate electrode 2 so that the semiconductor layer 4 covers over the gate electrode 2. A drain electrode 6 is selectively provided on a part of the semiconductor layer 4. A source electrode is provided on other part of the semiconductor layer 4 but separately from the drain electrode 6 so as to form an opening between the source and drain electrodes whereby a center part of the semiconductor layer 4 is shown by the opening for allowing a light to be incident through the opening into the semiconductor layer 4. The semiconductor layer 4 may be made of an amorphous silicon or a polysilicon so that when the semiconductor layer 4 absorbs a light having been incident, this generates carriers in the semiconductor layer 4. The semiconductor layer 4 may optionally be doped heavily with an n-type impurity to form an ohmic contact with the drain electrode 6. A reflective plate 5 serving as a pixel electrode is formed which extends over the gate insulation film 3. The reflective plate 5 is unitary formed with the source electrode which is positioned over the other part of the semiconductor layer 4. A color filter 10 is provided which extends over the reflective plate 5. An opposite electrode 8 is provided to be spaced apart from the reflective plate 5. The opposite electrode 8 is arranged which extends in parallel to the insulation substrate 1. The opposite electrode 8 is supported on a bottom surface of an opposite substrate 9. A liquid crystal is filled in the space defined between the opposite electrode and the reflective plate 5. The reflective plate 5 is made of a highly reflective material, for example, aluminum. The transistor formed over the substrate is an inverted stagger transistor.
Operations of the above transistor will subsequently be described. A gate voltage is applied to the gate electrode 2, then the drain electrode 6 is made conductive through the semiconductor layer 4 to the source electrode unitary formed with the pixel electrode in the form of the reflective plate 5. As a bias is applied across the drain electrode 6 and the pixel electrode, a current flows through the semiconductor layer 4 between the drain electrode 6 and the pixel electrode until the drain electrode 6 is the same in potential as the source electrode unitary formed with the pixel electrode in the form of the reflective plate 5. After the application of the gate voltage Vo to the gate electrode 2 is discontinued, no current flows between the drain electrode 6 and the pixel electrode. Charges remain stored in the pixel electrode. As a result, a voltage between the opposite electrode 8 and the pixel electrode is controllable by the transistor. The liquid crystal is optically changed by the variation in voltage between the drain electrode 6 and the pixel electrode. The color filter 10 allows a color display. The opposite electrode 8 and the opposite substrate 9 are made of an optically transparent material.
Incident light is transmitted through the opposite substrate 9 and the opposite electrode 8 into the liquid crystal 7. The incident light is then transmitted through the color filter 10 to reach the surface of the reflective plate 5 and reflected by the reflective plate 5. The reflected light is then transmitted through the color filter 10 and the liquid crystal 7 and further transmitted through the opposite electrode 8 and the opposite substrate 9. Other incident light is transmitted through the opposite substrate 9 and the opposite electrode 8 into the liquid crystal 7 and then transmitted through the opening between the drain electrode 6 and the source electrode to the semiconductor layer 4. The incident light is irradiated onto the semiconductor layer 4 and absorbed by the semiconductor layer 4. The absorption of the incident light causes generation of carriers, for example, electron-hole pairs in the semiconductor layer 4. Even if the pixel electrode was charged to apply an electric field to the liquid crystal 7, then the generation of carriers, for example, electron-hole pairs in the semiconductor layer 4 causes a current flowing between the pixel electrode and the drain electrode whereby the pixel electrode is discharged. As a result, the intensity of the electric field applied to the liquid crystal is dropped. This means all of the pixel electrodes are not charged and a reduced or almost no electric field is applied to the entire of the liquid crystal 7. This further means that contrast of the liquid crystal display is lowered.
It is, therefore, required to prevent the light from incidence into the semiconductor layer 4 or to shield the semiconductor layer 4 from the incidence of the light.
As described above, the reflective plate 5 is made of aluminum which has a high reflectivity but not chromium having a low reflectivity because if the reflective plate 5 is made of aluminum, then a sufficiently bright display can be obtained. If, however, the reflective plate 5 is made of chromium, then a dark display is obtained, which is unavailable. The reflective plate 5 made of aluminum however has a disadvantage in diffusion of aluminum into the semiconductor layer 4 made of polysilicon or amorphous silicon whereby the characteristic of the transistor is deteriorated.
Further more, the semiconductor layer 4 is made contact through a thin orientation film such as a polyimide film with the liquid crystal 7. The polyimide film is incapable of blocking ions in the liquid crystal 7 for allowing the ions passing through the polyimide orientation film to the semiconductor layer 4, for which reason the semiconductor layer 4 is contaminated with the ions in the liquid crystal 7. As a result, the characteristic of the transistor is deteriorated.
If, in order to settle the above problem, it was proposed to provide an inorganic insulation film over the semiconductor layer 4 for having the inorganic insulation film cover the semiconductor layer 4. The inorganic insulation film may comprise a silicon nitride film having been formed by a plasma enhanced chemical vapor deposition method. The inorganic insulation film cover the semiconductor layer 4 is required to selectively be removed for allowing opposite side regions to be shown so that source and drain electrodes may be formed on the opposite side regions. The insulation film may selectively be removed by, for example, photolithography but this additional process causes increase in the manufacturing cost of the reflective liquid crystal display.
In the above circumstances, it had been required to develop a novel reflective liquid crystal display free from the above problems in deterioration in display caused by generation of carriers, for example, electron-hole pairs in a semiconductor layer connecting the source/drain electrode due to incidence of light into the semiconductor layer.